With the rapid development of nanotechnology, accurate personalized treatments for tumor have attracted more attention to increase the therapeutic effects and reduce the side effects. The emerging theranostic systems incorporating different therapeutic and diagnostic imaging capabilities have become a new research hotspot. Magnetic iron oxide nanoparticles(IONP)have been widely used in theranostic systems due to their unique imaging properties, stable thermal performance, excellent biocompatibility and admirable surface modifiability. In this review, we analyzed the advantages of IONP in the diagnosis and the treatment of tumor, and detailedly introduced the relevant strategies and latest research progress, including magnetic resonance imaging(MRI), photothermal therapy, magnetic hyperthermia, and magnetic targeted drug delivery, etc. Finally, the potential application of IONP in the clinical tumor theranostics was proposed.